The present invention relates to an arrangement for adjusting the relative rotation position of a first camshaft of a reciprocating engine with respect to a second camshaft, or a crankshaft, that drives the first camshaft by a chain, and has a tensioning arrangement. The tensioning arrangement has two coaxial hydraulic pistons provided on the chain, and a helical spring which can tension the hydraulic pistons in opposite directions to one another against the loose run and the load run of the chain. The tensioning arrangement, as a whole, can be displaced transversely with respect to the chain.
An adjusting arrangement of the above-described type, which is used for changing the valve timing, is shown in German Patent Document DE-PS 40 06 910. A hydraulic tensioning and adjusting arrangement acts upon a chain by means of a first camshaft is driven by a second camshaft which is disposed in parallel to the first camshaft. This tensioning and adjusting arrangement includes two hydraulic pistons which are disposed concentrically with respect to one another. By means of a helical pressure spring, which is disposed in the hollow space between the hydraulic pistons, the hydraulic pistons are displaced in opposite directions with respect to one another, in which case the tensioning shoes fitted onto the hydraulic pistons are braced against the two chain runs. The outer hydraulic piston has a collar, to the two front faces of which pressure ducts are applied which can be acted upon by hydraulic pressure. If the front faces are alternately supplied with pressure, the runs which are each situated in the moving direction of the piston will be lengthened while the opposite runs will be shortened correspondingly. As tests have demonstrated, the system acts asymmetrically in both adjusting directions. This could be confirmed by calculations (See FIG. 1). As indicated by curve a, the adjusting force increases progressively for an unfavorable adjusting direction, and when it approaches the stretched-out position of the opposite run, it will approach infinity. Even when the camshaft is rotated by 4 degrees out of the symmetrical position, the adjusting force is approximately three times as high as for an adjustment in the opposite direction. The adjusting time comparison determined in tests indicates a similar behavior with a ratio of 0.6 to 0.2 seconds.
The cause of the more favorable behavior in the opposite direction is the fact that in this case the adjusting force of the piston does not act upon the run in addition to the prestressing spring but instead reduces the force of the run on the opposite adjusting side. It is immediately understandable that, for achieving the stretched-out position of the opposite run maximally an adjusting force in the amount of the spring prestressing is required.
An object of the invention is to provide a tensioning and adjusting arrangement of this type in such a manner that the adjusting force and the adjusting time is the same and as low as possible in both adjusting directions.
This and other objects are achieved by the present invention which provides an arrangement for adjusting the relative rotating position of a first camshaft of a reciprocating engine with respect to a second camshaft driving the first camshaft, comprising a chain drivingly coupled to the first camshaft and having a loose run and a load run, and a tensioning arrangement. This tensioning arrangement has two coaxial hydraulic pistons that engage the chain, and a helical spring coupled to the hydraulic pistons so as to tension the hydraulic pistons in opposite directions to one another against the loose run and the load run, the tensioning arrangement being transversely displaceable with respect to the chain for adjusting the second camshaft. Each hydraulic piston has a front face, said front faces being arranged so as to be alternately acted upon by hydraulic pressure in such a manner that a pressure force acting upon the hydraulic pistons is directed against the force of the helical spring acting upon the hydraulic pistons.
When, for the adjustment of the chain, the front faces of both hydraulic pistons can be acted upon independently of one another with hydraulic pressure in such a manner that the pressure force counteracts the spring force loading the hydraulic pistons, by means of the adjusting force, the effective spring force is reduced and the run to be shortened can easily be moved in the direction of the stretched-out position. The adjusting force and the adjusting time are the same in both adjusting directions and are very low. The tensile stress occurring in the chain runs during the adjustment and the forces on the bearings of the camshaft are also correspondingly low. Since the adjusting force rises only slightly along the torsion angle of the camshaft and particularly there is no rise toward infinity, the adjusting range determined by the geometrical quantities can be exhausted completely. When the geometry is unchanged, the adjusting force can be reduced considerably in the previous unfavorable direction. On the other hand, when the previous maximum adjusting force is accepted, the gained potential may be utilized for a reduction of the overall size.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.